Multi-channel spread spectrum communications system

ABSTRACT

A multi-channel spread spectrum communications system utilizes the combination of spreading codes to represent and transfer information. On the transmitter side, every block of data bits is divided into two sub-blocks of data bits. One sub-block is mapped to a number of spreading codes, while another sub-block is separated further into several smaller sub-blocks with each of them mapped into an M-ary phase. Each selected spreading code is modulated by the corresponding M-ary phase. On the receiver side, first determine the most likely transmitted spreading codes and their respective phases, then reverse map the set of estimated transmitted spreading codes into a block of bits and each of the phases into another block of data bits, and finally combine all these blocks to a single block of data bits.

FEDERALLY SPONSORED RESEARCH

[0001] Not Applicable

SEQUENCE LISTING OR PROGRAM

[0002] Not Applicable

FIELD OF THE INVENTION

[0003] The invention relates generally to multi-channel direct sequence spread spectrum communications system, and more particularly to using the elements of the power set of the set of spreading sequences to represent and transmit information in order to achieve a higher data rate without requiring more precision on A/D converter and more linearity on linear power amplifier.

BACKGROUND

[0004] Spread spectrum is a method of modulation that needs a transmission bandwidth usually substantially higher than data rate. In a direct sequence spread spectrum communications system, the transmitter modulates a data signal with a pseudo random chip sequence to generate spread spectrum signal. Usually the chip rate of the pseudo random sequence is much higher than the data rate and therefore the direct sequence spread spectrum communications system will take substantially wider bandwidth than the bandwidth needed by data signal itself. The spread spectrum signal is then transmitted over a communications media as a radio wave to a receiver. The receiver despreads the spread spectrum signal to recover the information contained in the received spread spectrum signal.

[0005] Having many advantages over other communications systems, direct sequence spread spectrum communications system is one of the major communications systems widely used in today's society. However, spread spectrum communications system has some disadvantages. One of the major disadvantages is low spectrum efficiency. Nowadays more and more applications require higher and higher data rate but the available bandwidth is both very expensive and limited. The low spectrum efficiency inherited in a spread spectrum communications system will greatly restrict its opportunity to be used in many high data rate applications.

[0006] Multi-channel direct sequence spread spectrum communications system is one of the attempts to increase the data rate within a given bandwidth and therefore to improve the spectrum efficiency of direct sequence spread spectrum communications system. However, in a regular multi-channel direct sequence spread spectrum communications system, the more channels, the more different signal levels. In order to have enough resolution, more linear power amplifier and higher precision A/D converter will be needed. Both of these factors will make cost greatly increased especially when the working clock is very high.

[0007] U.S. Pat. No. 6,324,209 to Don Li et al discloses a method of applying multi-channel technology in direct sequence spread spectrum communications system. Different from regular multi-channel direct sequence spread spectrum communications system in which each sub-channel transmits information simultaneously, U.S. Pat. No. 6,324,209 separates the spreading codes into several subgroups and at any time, transmit only one spreading code from each subgroup. In this way, it limits the possible number of signal levels and therefore the requirement on both A/D and linear power amplifier is under control. However, the data rate is still not high enough. In some case, it can not meet the requirements such as reasonable cost and limited transmission bandwidth set up by more and more high data rate applications.

[0008] Therefore, there is a need to further increase the spectrum efficiency of a multi-channel direct sequence spread spectrum communications system without increasing the requirement on linear power amplifier and A/D converter.

[0009] In a regular communications system, the information is represented and transmitted directly by each transmittable signal. Let S be the set of all these transmittable signals. The power set of the set S is a set denoted by 2^(S), whose elements are the subsets of S. In other words, 2^(S)={X|X⊂S}. If there are L components in S, then there will be 2^(L) elements in the power set 2^(S).

[0010] Suppose in a regular communications system, each element of S can carry 1 bit information. By transmitting all of the elements of S, one can send L bits information.

[0011] One can also send L bits information by alternative. Due to the fact there are 2^(L) elements in the power set 2^(S), one can correspond each element of the power set 2^(S) to a different L bits binary number. Since there are C_(L) ^(i) elements in 2^(S) with each element consisting of i components from set S, the element of 2^(S) on average consists of ${\left( {\sum\limits_{i = 0}^{L}{i \cdot C_{L}^{i}}} \right)/2^{L}} = {L/2}$

[0012] components from set S. In this alternative way, instead of transmitting L signals all the time, one can, on average, transmit only half of L signals.

[0013] Furthermore, instead of using all the elements of a power set to transmit information, one can use a particular subset of the power set to transmit information.

SUMMARY OF THE INVENTION

[0014] This invention is based on the fact that the number of the elements in the power set of a set is more than the number of the components in the set.

[0015] The primary objective of the invention is to achieve a multi-channel direct sequence spread spectrum communications system with a higher data rate without further requirement on linear power amplifier and A/D converter.

[0016] Another objective of the invention is to represent and transmit information by the elements of the power set of the set of spreading sequences instead of spreading sequences themselves.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawing figures depict preferred embodiments of the present invention by way of example, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements.

[0018]FIG. 1 is a block diagram of a communications system implementing the concept of power set.

[0019]FIG. 2 is a block diagram of the transmitter of a multi-channel direct sequence spread spectrum communications system implementing concepts of the present invention.

[0020]FIG. 3 is a block diagram of the receiver of a multi-channel direct sequence spread spectrum communications system implementing concepts of the present invention.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS

[0021] Detailed description of the preferred embodiment is provided herein. The embodiment illustrates a multi-channel direct sequence spread spectrum communications system. However, it is to be understood that the present invention may be embodied in many different ways. For those skilled in the art, it may be easy to modify the embodiment. For example, instead of using phase mapping device, one can use quadrature amplitude modulation (QAM) or pulse amplitude modulation (PAM). Therefore, specific details disclosed are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one to employ the present invention in virtually any appropriately detailed system, structure or manner.

[0022]FIG. 1 illustrates the general structure of a communications system transmitting information by the elements of a power set of the set of transmittable signals. For a set S having L components, the power set of the set S will have 2^(L) elements. Each element in the power set is a subset of the set S, consisting of 0 to L components from the set S. The huge difference between the number of components in a set and the number of elements in the power set of the set makes it possible on average to transmit more information under same or similar transmission conditions.

[0023] Suppose that there are N bits b₁, b₂, . . . , b_(N) to be transmitted and suppose that there are L codes in the set S. Let's arrange these L codes into order and call them code₁, code₂, . . . , code_(L). The power set 2^(S) of the set S is the set of all subset of the set S. The elements of the power set of the set S consist of none, some or all of the codes from code₁, code₂, . . . , code_(L).

[0024] The mapping device 110 maps a different block of N data bits b₁, b₂, . . . , b_(N) into a different element of the power set 2^(S). Let the mapping device 110 map a particular block of N data bits b₁, b₂, . . . , b_(N) into ch₁, ch₂, . . . , ch_(l), an element of the power set 2 ^(S), where ch_(i)≠ch_(j) if i≠j, ch_(i)=code_(i′), for 1≦i′≦l≦L and 1≦i≦L, and i′<j′ for ch_(i)=codes_(i′), ch_(j)=code_(j′) and i<j. The mapping is always possible as long as N≦L.

[0025] ch₁, ch₂, . . . , ch_(i) are transmitted through the transmission mechanism 120, which may include RF modulation, power amplifier and transmitting antenna at transmitter side and receiving antenna, low noise amplifier, automatic gain control and frequency down converted device at receiver side.

[0026] The output of transmission is connected to a matched filter bank 130. There are L matched filters in the bank with each one matching to one of signals represented by code₁, code₂, . . . , code_(L).

[0027] The maximum likelihood detection 140 determines which codes among code₁, code₂, . . . , code_(L) are most likely transmitted at the transmitter side. In other words, to determine ch′_(l), . . . , ch′_(l′). This detection could be carried out under some conditions such as false alarm probability and detection probability as well as some precondition on ch₁, ch₂, . . . , ch_(l).

[0028] The reverse mapping device 150 is used to find the likely transmitted bits b′₁, . . . , b′_(N) once ch′₁, . . . , ch′_(l′) have been determined. If there is no mistake, b′₁, . . . , b′_(N) should be identical to b₁, b₂, . . . , b_(N).

[0029]FIG. 2 illustrates the transmitter of a multi-channel direct sequence spread spectrum communications system implementing the concept of power set.

[0030] In a multi-channel direct sequence spread spectrum communications system such as the one described in U.S. Pat. No. 6,324,209, each of the transmitted spreading codes may carry information by itself. Suppose in the system described by in U.S. Pat. No. 6,324,209, there are 64 codes separated into 8 groups with 8 codes in each group. Further suppose each code can have 16 different phases. Then during each symbol period, 8·(3+4)=56 bits could be transmitted. Using the concept of power set, it is possible to transmit more bits with only 8 codes transmitted at any time.

[0031] The total combinations to choose 8 codes from 64 codes are C₆₄ ⁸=4,426,165,368. Since C₆₄ ⁸>2³²=4,294,967,296, from a given 8 codes, 32 bits can be obtained from reverse mapping. Also 4 bits can be obtained from the phase associated with each code of the 8 codes. Therefore a total of 32+8*4=64 bits can be transmitted.

[0032] There are 2⁶⁴−C₆₄ ⁸ extra elements in the power set 2^(S) which have not been used yet. These extra elements could be used for signaling, such as using one extra element to indicate no symbol repetition, another extra element to indicate 1 symbol repetition, a third extra element to indicate 2 symbol repetitions. A transmitter transmits an extra element corresponding to the desired symbol repetition. A receiver, after detecting the extra element, is able to find the intended symbol repetition set up by the transmitter.

[0033] Among these extra elements, there are 4,426,165,368−4,294,967,296=131,198,072 elements with 8 codes in each element and there are ${\sum\limits_{i = 1}^{7}C_{64}^{i}} = {704,494,192\quad {elements}}$

[0034] 704,494,192 elements with 7 or less codes in each element. If only the extra elements with exactly 8 codes in each element are used for signaling, the energy associated with each code of the element for signaling will be equal to the energy associated with each code of the element used for information transmission. If the extra elements with less than 8 codes in each element are used for signaling, the energy contained in each code for signaling will be larger than the energy in each code for information transmission. Therefore, more reliable signaling can be provided when the extra elements with fewer codes are used for signaling.

[0035] Suppose on the data bus 205, there are 64 bits needed to be transmitted simultaneously. The bit separation device 210 separates bus 205 into two data buses 215 and 220 with each bus having 32 bits. The mapping device 225 maps the 32 bits on data bus 215 into 64 bits with 8 bits set to 1 and the other bits set to 0.

[0036] There are many way of mapping 32 bits into 64 bits with 8 bits set to 1 and the other bits set to 0. The simplest mapping algorithm for the mapping device 225 could be done as following. First, list the entire binary table for 64 bits in ascending order. Then delete all the items which do not include exactly 8's 1 and 56's 0. Next, start from beginning, map each of the 64 bits of the remaining first 2³²=4,294,967,296 items to a 32 bits sequence 0 . . . 000, 0 . . . 001, 0 . . . 010, 0 . . . 011, . . . 1 . . . 111 correspondingly.

[0037] Set 235 consists of 64 codes denoted C₁, . . . , C₆₄. The code selection device 240 select codes according to corresponding bit on the data bus 230. If a bit on the data bus 230 is 1, the corresponding code will be selected. Exactly 8 codes will be selected and they are denoted as C′₁, C′₂, . . . , C′₈.

[0038] Data bus 220 is connected to bit separation circuit 250, where the 32 bits are further divided into 8 groups with each group having 4 bits on each data bus 255. The phase mapping device 260 maps each of the 4 bits on data bus 255 into a complex number which corresponds to one of the 16 possible phase of a 16PSK signal. Each of these 8 complex numbers is multiplied with C′₁, C′₂, . . . , C′₈ by one of the multipliers 245 respectively. The complex signal combiner 265 adds all the 8 complex products of the output of multipliers 245 together and then separates the real signal and image signal as I and Q. The I signal is multiplied by PN code 270, modulated by cos(ωt); Q signal is multiplied by PN code 270, modulated by sin(ωt). Both I and Q are coupled together, then sent to power amplifier 305 for amplification, and then to antenna 310 for transmission.

[0039]FIG. 3 illustrates the receiver of a multi-channel direct sequence spread spectrum communications system implementing the concept of power set.

[0040] The signal enters the receiver through the antenna 405 and goes through the voltage controlled amplifier (AGC) 410. The output of AGC 410, is multiplied by cos(ωt) 415 at multiplier 420 and by sin(ωt) 425 at multiplier 430. The phase rotator 435 is used to correct the phase error. The PV code from the PN generator 440 and all the spreading codes C₁, . . . , C₆₄ are fed to the matched filter bank 445, which is used to find that how closely the input signal is matched with each of spreading codes C₁, . . . , C₆₄ scrambled by PN code. The maximum likelihood detection device 450 will determine 8 most likely transmitted codes. Device 450 has 8 groups of output with each group of output consisting of an I component, a Q component and the index of corresponding code. Index i<Index j for i<j.

[0041] The 8 indexes with one from each group are fed to the reverse mapping device 455 to obtain 32 data bits, which should be same as the one on data bus 215 in FIG. 2 if no error is made.

[0042] The I and Q signals from each group are fed to one of the 16PSK slicers as denoted by 460 a through 460 h, which selects the most likely transmitted phases among the 16 possible phases. The outputs of these phase slicers are sent to the reverse phase mapping devices 465 a to 465 h to obtain the remaining 4 bits from each. The bit combination device 470 combines the 4 bits from each of the 8 reverse phase mapping devices to form demodulated 32 data bits, which should be identical to the bits on the data bus 220 of FIG. 2.

[0043] The combiner 475 combines the 32 bits form the bit combination 470 and the 32 bits from the reverse mapping device 455 to form a total of 64 demodulated bits. If no error occurs during transmission, the demodulated bits should be identical to the 64 bits on the data bus 205 in FIG. 2. 

What is claimed is:
 1. A multi-channel communications system, comprising: a mapping device at transmitter for mapping a block of data bits into an element of the power set of the set of transmittable signals; a detection device at receiver for determining the likely transmitted element from the power set of the set of transmittable signals; and a reverse mapping device at receiver for reverse mapping the likely transmitted element of the power set of the set of transmittable signals into a block of data bits.
 2. A multi-channel communications system as in claim 1, wherein the transmittable signals comprises spreading signals.
 3. A multi-channel communications system as in claim 2, further comprising: a bit separating device for dividing a block of data bits into several smaller blocks of bits; a plurality of submodulators for modulating each of the spreading codes of an element of the power set with the signal mapped from the corresponding smaller block; a plurality of subdemodulators for demodulating the signal associated with each of the spreading code of an element of the power set into a block of data bits; and a bit combining device for combining a plurality of blocks of data bits together.
 4. A multi-channel communications system as in claim 3, wherein each of the submodulators comprises an M-ary PSK modulator.
 5. A multi-channel communications system as in claim 3, wherein each of the submodulators comprises a QAM modulator.
 6. A multi-channel communications system as in claim 3, wherein each of the submodulators comprises a PAM modulator.
 7. A multi-channel communications system as in claim 3, wherein each of the subdemodulators comprises an M-ary PSK demodulator.
 8. A multi-channel communications system as in claims 3, wherein each of the subdemodulators comprises a QAM demodulator.
 9. A multi-channel communications system as in claims 3, wherein each of the subdemodulators comprises a PAM demodulator.
 10. A method of transmitting and receiving a block of data bits comprising: separating the block of data bits into a plurality sub-blocks of data bits; mapping one of the sub-blocks into an element of the power set of the set of spreading codes; mapping each of the rest sub-blocks into a signal corresponding to the data bits in the sub-block; multiplying each of the spreading codes of said element with the corresponding signal to form a product; combining all the products to obtain a summation; modulating the summation with carrier and transmitting over a link; demodulating the received signals by multiplying it by the signals from local oscillator; detecting the most likely transmitted element of the power set of the set of spreading codes and signal associated with each of the spreading codes of the element; reverse mapping the most likely transmitted element into a first block of data bits; reverse mapping the signal associated with each of the spreading codes of the most likely transmitted element into a second block of data bits; and combining the first block of data bits with a plurality of second blocks of data bits to form a new block of data bits.
 11. A method as in claim 10, wherein mapping each of the rest sub-blocks into a signal, the signal comprises a complex number corresponding to the phase represented by the sub-block of data bits.
 12. A method as in claim 10, wherein mapping each of the rest sub-blocks into a signal, the signal comprises a complex number generated by QAM modulator corresponding to the sub-block of data bits.
 13. A method as in claim 10, wherein mapping each of the rest sub-blocks into a signal, the signal comprises a number generated by PAM modulator.
 14. A method as in claim 10, wherein reverse mapping the signal associated with each of the spreading codes, the signal comprises a complex number corresponding to the phase associated with corresponding spreading code.
 15. A method as in claim 10, wherein reverse mapping the signal associated with each of the spreading codes, the signal comprises a complex number corresponding to the real amplitude and imagery amplitude associated with the corresponding spreading code.
 16. A method as in claim 10, wherein reverse mapping the signal associated with each of the spreading codes, the signal is the magnitude associated with the corresponding spreading code.
 17. A method of automatic rate detection comprising: associating different symbol repetition to a corresponding extra element of the power set of the set of transmittable signals; transmitting the extra element corresponding to the desired symbol repetition; detecting the transmitted extra element; and finding the intended symbol repetition by checking the repetition associated with the received extra element. 